1. Field of the Invention
The invention relates to an apparatus for simultaneous OTDM demultiplexing, electrical clock recovery and optical clock generation, and to an apparatus for optical clock recovery, using a traveling-wave electroabsorption modulator.
2. Description of Related Art
Demultiplexing, add/drop, regeneration, clock recovery and synchronization are the key functions required within an optical time-division multiplexing (OTDM) network node. The ability to demultiplex a single channel from a high-speed OTDM data stream and achieve clock recovery to ensure correct synchronization of all data channels is significant at a network node for further switching and detection processes. It is desirable to simultaneously perform demultiplexing and clock recovery using a single device to reduce the cost and complexity of the network node. However, the possible problems of the ambiguity and the crosstalk could occur in the single one device that is employed for multiple different purposes at the same time.
Examples of prior attempts to implement simultaneous demultiplexing and clock recovery were made by B. Mikkelsen, G. Raybon, R.-J. Essiambre, “160 Gb/s TDM Transmission Systems,” Paper 6.1.1, pp. 125–128, ECOC 2000, and J. Yu, K. Kojima, N. Chand, “Simultaneous Demultiplexing and Clock Recovery of 80 Gb/s OTDM Signals Using a Tandem Electro-absorption Modulator,” PDI, pp. 2–3, LEOS 2001.
The ability to generate an optical clock from a high-speed optical time-division multiplexed (OTDM) data stream and ensure correct synchronization is significant in an OTDM network node for all-optical 3R (i.e., re-generation). However, usually, it is necessary for realizing optical clock recovery that an additional photodetector is required to detect the data information and an additional pulse generator is required to produce the optical clock.
FIG. 15 shows a third example of the related art where discrete components for optical clock recovery are used. Data light, which is light having data therein, comes in through a fiber to a clock recovery device (CR). The CR has a function of a photodetector and produces an electrical clock signal. The electrical clock signal is supplied to an external intensity modulator (MOD). One example for the MOD is a TW-EAM. On the other hand, a laser diode (LD) radiates a laser beam to the MOD where the laser beam is modulated by the electrical clock signal. As a result, the MOD generates an optical clock signal.
FIG. 16 shows a fourth example of the related art where discrete components for demultiplexed data recovery are used. Data light signal having a bit rate of 40 Gb/s data stream impinges on a photodetector (PD) and an electroabsorption modulator (EAM). In response to the data light, the PD produces a photocurrent to be supplied to a phase-locked loop (PLL), which in turn outputs an electrical clock. The electrical clock has a frequency derived by dividing a fundamental frequency of the bit rate of the data stream by, for example, 4, namely 10 GHz that is applied to the EAM. The EAM produces a bit rate of 10 Gb/s data stream. Also in the fourth example, discrete components of the PD and the EAM are used.
Clock recovery for a signal other than a traveling wave can be explained by, for example, Japanese Patent Publication Hei 11-38371 disclosed Feb. 12, 1999 (Japanese Patent Application Hei 9-189748 filed Jul. 15, 1997). The Publication uses pulsed light as an input signal to be fed into a semiconductor light modulator. The modulator generates a photocurrent that is applied to a circulator. Regarding pulsed light other than a traveling wave (TW), a circulator is used, but is expensive. On the other hand, a traveling wave does not need an expensive circulator. In addition, a traveling wave can be used to produce an electrical clock that is accurately synchronized with the traveling wave.
Although a TW-EAM is used as a MOD, the TW-EAM is used to only absorb and pass incoming light, i.e., is used to modulate the intensity of the incoming light.
Thus, there is a need for one device with respect to a traveling wave that has the functions of both a photodetector and a modulator, and extracts a clock in incoming data light in addition to absorption and passing of the data light.